The present disclosure relates to semiconductor structures and methods of fabricating the same. More particularly, the present disclosure relates to heterojunction bipolar transistors and methods of fabricating the same.
The heterojunction bipolar transistor (HBT) is a type of bipolar junction transistors (BJT) which uses differing semiconductor materials for the emitter and base regions, thus creating a heterojunction. The HBT improves on the BJT in that the HBT can handle signals of very high frequencies, up to several hundred GHz. HBTs are commonly used in ultrafast circuits, mostly radio-frequency (RF) systems, and in applications requiring high power efficiency, such as RF power amplifiers in cellular phones.
Heterojunction contacts are of interest in bipolar junction transistors particularly at the emitter junction for increasing the gain of the transistor. Conventionally, heterojunctions are formed by epitaxial growth techniques such as metal-organic chemical vapor deposition (MOCVD), which are expensive and/or require high temperatures, typically greater than 600° C. Depending on the application, the high growth temperature may have any or all of the following drawbacks: degradation of minority carrier lifetime, creation of structural defects, undesired impurity diffusion resulting in junction widening, relaxation of strain or generation of undesired strain which may result in buckling or delamination.